A thin film transistor based on a thin film deposition technology, which is mainly used as a backplane element of a flat panel display, has been continuously developed. Recently, an oxide semiconductor thin film transistor using a metal oxide semiconductor has received a lot of attention.
In order to manufacture the thin film transistor, a process of forming an oxide film that is a channel layer and a process of forming a source region and a drain region by adding hydrogen to a part of the oxide film are required.
As illustrated in FIG. 1, a self-aligned thin film transistor 10 according to the related art includes a substrate 100, an active layer 110, a doping source film 130, a gate insulating film 140, a gate electrode 150, an interlayer insulating film 160, a source electrode 172, and a drain electrode 174.
As illustrated in FIG. 1, in the self-aligned thin film transistor according to the related art, a source region 122 and a drain region 124 are formed by depositing a doping source film 130 in a region in which the source electrode 172 and drain electrode 174 contacts the substrate 100.
However, in the method, a diffusion of hydrogen used for the doping occurs during subsequent processes, such that a doping region (i.e., a doped region) may be formed in a region wider than a region that is actually needed. The wide doping region penetrates into a bottom portion of the gate insulating film 140 through the gate electrode 150 to hinder an effect of removing the overlap between the gate and source regions and between the gate and drain regions, which is an advantage of the self-aligned structure. The expansion of the doping region disadvantageously affects the self-aligned device structure. That is, when the device length of the thin film transistor is short, a short channel effect is generated due to the expanded doping region and in more severe cases, the doping regions are connected with each other within the channel such that the operation of the device may be hindered.
Therefore, a need exists for a structure for effectively preventing the expansion of the doping region. However, there is a need to sufficiently consider subsequent processes because the expansion of the doping region continuously occurs due to the subsequent thermal process of the device.